1. Field of the Invention
The invention relates to color copiers and printers and more particularly to automatic adjustment of parameters influencing the output copy color balance, color fidelity, and tone reproduction.
2. Description of the Prior Art
In electrophotographic copying a color original document, several factors inhibit perfect and constant color reproduction in terms of color balance, color fidelity, and tone reproduction. These factors include variation in the color and intensity of the light source used to illuminate the document, variation in the spectral reflectance of the different colorants of the document, non-ideal color separation filters, variation in the photoresponse of the photoconductor, variation in the toning contrast in the different color development stations, and unwanted absorptions in the colored toners.
Prior art systems attempt to diminish the adverse effects of these factors using manual or automatic set up systems, or a combination of both. In typical manual set up systems, a skilled operator examines the output reproduction (copy or print) and the corresponding input and output density (D.sub.in -D.sub.out) curves for red, green, blue, and black. Based on experience with the equipment, the operator determines adjustments to process control parameters, such as initial voltage V.sub.O, exposure E.sub.O, and development bias V.sub.b. Several iterations of adjustment may be required to achieve acceptable color reproductions, in terms of color balance, color fidelity, and tone reproduction.
The set up procedure is complicated by the fact that a process control parameter adjustment which is favorable to one region of the reproduction may be detrimental for another. For example, a particular color may be too dark at high density levels and too light at low density levels. Accordingly, an adjustment to lighten the color to correct for high density errors would compound the low density error.
During set up, skilled operator will generally image a neutral density step tablet and adjust the process for hue neutrality of the reproduction. After achieving reasonable neutrality, the operator will adjust for good tone reproduction (i.e., good light-to-dark progression, contrast, and absence of abrupt density changes between density steps). Finally, the operator will check and adjust for neutrality again, all this in an iterative procedure until satisfied with the overall resultant reproduction quality. The set up inevitably involves compromises over all the color areas in the print, and even a highly skilled operator may be unable to achieve acceptable color reproduction within a reasonable time period.
Color copiers and printers are known which include automatic set up means for adjusting one or more of the process control parameters affecting the output color and density. Such automatic adjustment is typically based on density measurements of toned test patches for each color separation independently of the other color separations. However, the human observer is critical not only the appearance of individual colored areas taken individually, but also judges overall color and tone scale quality by the relationship of one color to another. For example, a slight hue error might be acceptable if the error is uniform over the entire image, but it would be unacceptable if the error is either (1) in one direction in some colors and in another direction in other colors or (2) in one direction for some densities and in another direction for other densities of the same color.